An Evaluation of Anthropogenic Trace Metals in the Western Basin of Lake Superior

Abstract

Lake Superior sediment cores were collected and analyzed for a suite of trace metals in the solid and aqueous phase. There have been few prior studies of redox chemistry of trace metals and the effects of early diagenesis on metal in Lake Superior. In this study, redox chemistry of Fe, Mn and Co is examined, in addition to the sediment profiles of Cu, Pb, Zn and Ba. The interaction between the solid and aqueous phase redox elements is consistent with those found in other studies. The redox elements show a classic distribution, a distinct solid phase peak with a corresponding rapid increase in aqueous phase concentration. Historical events are discerned in the sediment record: the beginning of Cu mining in Michigan; and the rise and fall of alkyl lead consumption in the U.S. A mathematical model is used to predict early diagenetic concentrations of the redox elements (Fe, Mn and Co) in solid and aqueous phase assuming simple one-dimensional diffusion. The results of the model are compared to the actual vertical profiles of these metals. Trace metal fluxes of Cu and Zn into and out of the sediments are calculated. The net flux of Zn shows significant regeneration and release into the water column. Sedimentation rates and core chronologies are based on 210Pb analyses. The 100-year average sedimentation rates range between 0.011 and 0.018 g/cm2/yr (0.046 to 0.059 cm/yr), and are compared to others reported for Lake Superior. X-ray diffractometry is used to identify the presence or absence of taconite tailings and the general clay lithology at each site. Three of seven sites are affected by taconite mining. Taconite tailings were discharged into the lake beginning in 1956, therefore the first appearance of tailings in the sediment column can be used to calculate a rudimentary sedimentation rate, which ranged between 0.085 and 0.098 cm/yr.